1. Field of the Invention
This invention pertains generally to printed circuit board electronic devices, and more particularly to electrical interconnects between board areas, and most particularly to printed circuit board impedance matching for broadband microwave devices.
2. Description of Related Art
High frequency electronic circuits, e.g. microwave circuits, require the propagation of electromagnetic signals along various paths. Various forms of transmission lines and other waveguides are used in these circuits. In a transmission line or waveguide, the electromagnetic field is confined to an area around physical elements along the transmission path (in contrast to antennas where the electromagnetic field is radiated into space).
Transmission lines use a physical configuration of conductors and dielectrics to direct a signal along a desired path. Most transmission lines use two conductors, a signal carrying conductor or trace and a ground conductor or return path.
Common transmission lines include microstrip lines, striplines, and coplanar lines (or waveguides). A microstrip line has a trace separated from a ground plane by a dielectric layer. A stripline has a trace embedded in a dielectric layer which is sandwiched between two ground planes. A coplanar line (or waveguide) has a trace separated from a pair of ground planes, all on the same plane atop a dielectric layer; an additional ground plane may also be added on the opposite side of the dielectric layer.
A microstrip is a particular type of electrical transmission line that is highly preferred for microwave circuits because of its simplicity and ease of fabrication. The microstrip can be fabricated with printed circuit board (PCB) technology. The microstrip is made up of a conducting strip or trace separated from a ground plane by a dielectric layer or substrate. The trace and ground plane are conductors, typically metal. The substrate is made of a dielectric material. An upper dielectric, typically air, surrounds the trace and substrate surface.
The microstrip can be used not just as a transmission line but also to form other circuit elements. Various microwave components, including antennas, couplers, filters, and power dividers, can be formed from a microstrip, as a pattern of metallization on the substrate that produces the entire microwave device (circuit).
Microstrip lines, as well as other transmission lines and other circuit components, have a characteristic impedance. The impedance of a microstrip line is a complex function of its geometry, dimensions, and material properties. The distance between the trace and ground plane affects the impedance.
Impedance matching at the interconnects between circuit components, e.g. between a coaxial cable and an electronic device, is necessary for efficient coupling. By closely matching the output impedance of the cable to the input impedance of the device, electrical signals are transmitted from the cable to the device without substantial reflection.
One particular type of circuit where an electrically seamless connection is difficult is between a broadband microwave (millimeter wave) device, such as a patch antenna, and a coaxial connector/board connection. To achieve broadband performance for the patch antenna, the ground plane beneath the patch antenna must be on a substrate with a thickness that is a sizeable fraction of the wavelength of the desired frequency of operation. However, to energize the antenna by a coaxial cable interconnect, the ground plane for a transmission line connecting the microwave (millimeter wave) device and the connector is only feasible on a substantially thinner substrate. In general circuits are fabricated on substrates (PCBs) of constant thickness.
One common area where this problem exists is in the mobile wireless markets. There a small number of broadband path radiators, necessarily on thick substrates, are interconnected to RF feeding networks requiring thinner substrates for impedance control.
Accordingly it is desirable to provide broadband microwave electrical interconnect between board areas containing different heights in surface-to-ground plane (return path) location.